The heavy machinery running at a Fresno, Calif., meat processing plant drowned out the gunshots as an employee methodically killed two co-workers execution style and wounded two others before he took his own life.

The shooting erupted around 8:27 a.m. Friday at Valley Protein.

"This is a meat processing plant, which is very noisy, but more importantly several of the victims had ear protectors, ear plugs in their ears and would not have heard the gunshots from their co-workers being shot and killed," Fresno Police Chief Jerry Dyer said at a news conference Tuesday.

The deceased were identified as Salvador Diaz, 32, and Manuel Verdin, 34.

Arnulfo Conrriquez, 28, was in serious condition after getting shot in the neck, while Fatima Lopez, 32, was in stable condition after suffering a wound to the lower back.

Investigators now face the task of understanding what drove 42-year-old Lawrence Jones to commit an act of workplace violence.

Lawrence tried to shoot a fifth co-worker execution style, police told ABC News' Fresno affiliate KFSN. When he realized he was out of bullets, Jones went outside, reloaded and shot himself in the head.

Jones was found lying on a sidewalk with a handgun nearby. He later died at the hospital.

At 4:58 a.m. last Friday, Jones, who had worked at the plant for 14 months, clocked in for work.

"According to one of his co-workers, he did not appear to be himself when he came to work," Dyer said.

Less than four hours later, Jones, who Dyer said had an "extensive criminal history," opened fire on his co-workers.

One surviving victim was in critical condition, while another was listed as serious.

A recording at Valley Protein said the business remained closed "due to an emergency."